What is the difference between deleting and unsubscribing a subscriber?

You may see two options in your account to remove subscribers. You can either delete a subscriber, or mark them as unsubscribed from all. It's best practice to unsubscribe a subscriber who you no longer want to receive messages from you. The benefit to unsubscribing them is that they will not receive future messages from you. Even if they're imported on a future list, they will not receive messages from you because they're flagged globally as unsubscribed to all. They'll also no longer count as an "active" subscriber on your lists.

You can mark someone as unsubscribed by following these steps:

1. Go to All Subscribers
2. Search for an email address and double click to edit the subscriber.
3. Check the box next to Unsubscribe from All to globally unsubscribe this person in your account. 

However, if you select to delete a subscriber, it will remove the email address and all their data (like unsubscribe preferences, lists they are on, send history, custom field values, etc). It is not recommended to delete subscribers from your account, especially if the subscriber has previously opted-out of your messages. If you delete a subscriber, then you can accidentally import them as a brand new subscriber in the future. That's why it's typically recommended to unsubscribe someone who wants to be removed, instead of deleting them.